A flanged device is ordinarily bolted by a set of bolts and nuts to assemble into a high pressure system, e.g., a valve in pipeline and the like. The seal is critical in high pressure devices. Another example is found in the flange connections of an assembled blow out preventer stack, often called the BOP stack. A method and a test device are disclosed so that a seal can be established at the ring or seal gasket located between two opposing flanges by means of a hydraulic force transmitted from the test piston to the seal area between the flanges.